Stress is a physiological response of the sympathetic nervous system to environmental pressures. If left unchecked it can lead to patho-physiologies such as hypertension and diabetes. Chromaffin cells of the adrenal medulla are a primary endocrine output of the sympathetic nervous system and secrete a host of transmitter molecules, including catecholamines and peptide transmitters, through the fusion of dense core secretory granules. At basal firing rates, set by the sympathetic tone, catecholamine output regulates homeostatic processes such as enteric function, vascular tone and insulin secretion. Stress-mediated sympathetic activation leads to elevated catecholamine secretion, increasing cardiac output and glucagon levels. Stress also evokes peptide transmitter release, including enkephalin which acts as an analgesic and allows the organism to focus on escape and defense. Our goal is to understand how chromaffin cells regulate the release of catecholamine and signaling peptides under physiological basal-firing and stress activation. Cellular mechanisms for catecholamine release have been studied, but peptide transmitter release is less well understood under these conditions. In this proposal we will test that, under basal firing, chromaffin granules undergo a restricted fusion with the cell surface and selectively release catecholamine. We will test that stress-activation induces a full fusion and collapse of the granule leading to release of both catecholamine and peptide cargos. We will employ electrophysiological, electrochemical, fluorescence imaging, immunocytochemical and biochemical approaches. We will test for exocytosis of catecholamine and protein cargo from chromaffin granules under basal-firing and stress-activated states. We will test the roles of key molecules in the exo-endocytic mechanism and we will measure the degree granule collapse into the cell surface during the exo-endocytic cycle. This data will provide a fundamental understanding of the cellular and molecular mechanisms for the endocrine stress response in chromaffin cells.